1. Field of the Invention
The present invention relates generally to sash windows, and more particularly to a single-hung or double-hung window having at least two sashes supported in vertical juxtaposition within a window frame.
2. Description of Related Art
Sash windows adapted to be mounted in an opening in the exterior wall of a commercial or residential building are known as single- or double-hung windows having a pair of upper and lower sashes. A typical example of such known sash windows is described in U.S. Pat. No. 6,826,871 B2 which comprises a window frame including a head member, a sill member and a pair of jamb members connected together into a rectangular configuration, and a pair of sashes each having a sash frame including a top rail, a bottom rail and a pair of side rails connected together into a rectangular configuration, and a panel member of glass disposed within the sash frame. The window frame members and the sash frame members are formed from an elongated strip of extruded highly rigid plastic such as PVC (polyvinyl chloride). Four such elongated strips are assembled together into a rectangular frame by welding together mating ends thereof.
In many cases, joining between the head member and each jamb member is accomplished by first cutting an end of each of the head and jamb members at an angle of 45 degrees relative to a longitudinal axis of the head or jamb member and subsequently welding together the cut or beveled ends of the head and jamb members to form a miter joint. With this joining structure, since the beveled mating ends of the head and jamb members being welded together have a common cross-sectional shape, a sufficiently large structural strength can be obtained.
On the other hand, joining between the sill member and each jamb member is accomplished by first cutting an end of the jamb member at right angles to the longitudinal axis of the jamb member and subsequently welding the cut end of the jamb member to an upper surface of the sill member to form an end-butt joint. This joining structure is employed mainly for a reason that a sill member arranged to extend over the entire width of the window frame is preferred from an aesthetic viewpoint as compared to a sill member miter-joined with each jamb member in the same manner as the head member. The end-butt joint used for joining each jamb member to the sill member cannot provide a strength comparable to that of the miter joint formed between the head member and each jamb member, however, the miter joint when used as a joint between the sill member and each jamb member would reduce the commercial value of a finished sash window due to its less preferable appearance.